Thanks to Famitsu via GC-Inside, the possible plotline for Mario Sunshine is finally revealed. Thankfully, we won't have to rescue Princess Peach this time-- or will we?

Follows is our rough translation:

"Princess Peach was vacationing on Dolpic [Dolphin?] Island when many large scribbles appeared [on the island]. Mario vows to bring those responsible for this crime to justice and clean up the island in the process. Until then, Princess Peach can't finish her vacation! Mario must track down the culprit making use of his new waterpump backpack."

Source: GC-Inside / Famitsu